Process and machine for packaging products with stretchable thermoplastic film

ABSTRACT

Means are provided to ensure that during the stage of formation through the tube- forming and longitudinal welding means ( 1, 18 ) the wrapping is subjected to longitudinal stretching which causes it to adhere intimately in a direction transverse to the product and this condition is ensured by means which bring about a particular difference in velocity between the jaw ( 19, 119 ) which supports the tubular wrapping and causes it to advance and the means ( 15, 115, 17 ) which feed the film to the tube-forming means ( 1 ) which also remain suitably active during the subsequent stage in which the tubular wrapping is seized posteriorly by the appropriate posterior closing jaw ( 119, 19 ), all in such a way that the wrapping film runs over the said tube-forming means in an almost continuous movement to prevent localised appearance defects in the packaging and to assist operation of the longitudinal welding means ( 18 ) which are then able to operate substantially continuously.

The invention relates to structural and functional improvements to the process and machine described in document WO 03/051715 A1 of the same applicant, to which the most extensive reference is made. For a better understanding of the aims of the invention it must be remembered that the process described in the above cited document provides for unwinding the stretchable film from a feed spool, inserting that film in a horizontal mandrel to form a welded leaktight wrapping tube with overlapping longitudinal edges, holding and sealing the forward end of the said tube with jaw means by making two welds and cutting in-between, inserting the product which is to be packed into the inlet opening of the said film-tube-forming mandrel and positioning it at the anterior closed extremity of that tubular packaging which is caused to advance horizontally as an extension of the said forming mandrel by the said anterior jaw and supporting means. When a sufficient length of the tubular wrapping with the product within it, which is in proportion to the dimensions of that product, has left the forming mandrel, the tube wrapping is stopped and decelerated transversely by opposing clamps which clamp it for example at the discharge outlet from the forming mandrel complex, while the said anterior jaw means continue to advance the length of the progressively elongating tubular wrapping so as to stretch it longitudinally by a certain amount, this then shrinking by reaction transversely and adhering closely to the product located within it. When the product has completely exited from the tube-forming mandrel complex and is sufficiently distant therefrom, the posterior extremity of the stretched packaging is gripped transversely by a second clamping device which forms two transverse welds and a cut between them to seal and hold the rear end of the packaging with the product, while the anterior clamping, welding and cutting device opens, reverses the horizontal movement to return to the cycle start position and leaves the packed product supported on comb means associated with the lower component of the active jaw, welding and cutting device, which holds the end of the new length of tube of film at the outlet from the forming mandrel to repeat a new working cycle as described. At the appropriate time the said clamps which immobilised the film at the discharge opening of the forming mandrel complex draw back to allow the new tubular packaging film to advance, while the pack formed in the previous cycle is moved away from the active transverse jaw, welding and cutting device, and while that device is at its opening stage the pack is set down on a comb conveyor which carries it away.

Transverse clamping of the tubular film above and below the discharge opening from the tube-forming mandrel, through the action of the corresponding upper and lower transverse clamps, leaves the side portions of the tubular film free and these transmit imbalanced tensions to the film both upstream and downstream, with the danger that the proper position of the film on the said mandrel and also the spatial relationship with the longitudinal welding means is altered. In order to overcome these disadvantages, the clamps in front of the same have been removed and it is provided that the advance of the packaging film is stopped temporarily, stopping the return rollers located upstream of the tube-forming mandrel which act on the film transversely when the latter is still stretched, before it reaches the inlet opening of the said mandrel. One of these rollers is a rubber-coated roller and means which can stop it under control, for example drive means with a brake or an electromagnetic brake, are associated with it, and when this roller is stopped it forms an effective clamp which engages the film transversely and uniformly over its entire length. The longitudinal stretching to which the length of film which has left the forming mandrel is cyclically subjected and within which the tray with the product is positioned uniformly discharges its own energy over the entire length of film between the stretching clamps and the said rubber-coated roller, so that the film itself is not subjected to any appreciable longitudinal displacements which would have an adverse effect on the longitudinal welding means.

Through this arrangement it has been found that use of the said film-braking means upstream of the tube-forming mandrel often does not make it possible to deliver all the quantity of film which is effectively required by the sealing and stretching movements of the stretching and sealing jaws and transverse cutting of the packaging. The film tended to remain on the tube-forming mandrel, required difficult intermittent operation of the longitudinal welder and created friction when the film was first detached from the mandrel which substantially adversely affected the stretching and deformation gradient of the film and which could result in local laceration of the film and/or unsightliness in the packaging.

In order to overcome these disadvantages it has been found suitable to control the said rubber-coated roller through a motor with electronic speed and phase control and to operate that motor in a close relationship with the pull exerted by the pulling jaw, which is cyclically activated. The characteristics of this arrangement and the advantages deriving therefrom will be apparent from the following description of a preferred embodiment of the same, which is illustrated purely by way of a non-restrictive example in the figures of the appended sheets of drawings, in which:

FIG. 1 is a diagrammatical view in lateral elevation of the machine in question incorporating the improvements according to the invention,

FIGS. 2, 3, 4, 5 and 6 show diagrams illustrating the different modes of operation of the said film feed motor in relation to the horizontal translational and sealing movement of the one of the two drawing jaws which is cyclically activated.

In FIG. 1 it will be seen that the machine comprises a forming mandrel 1, of the so-called sling type, which is fixed or possibly adjustable, located in an almost horizontal position, formed by angled parts supported on supports 101 and through which there passes longitudinally a tubular guide 1′ supported by external parts 14 which has for example a rectangular cross-section and is such that product P being packed, which is normally intended to be placed in a tray, can pass through it with sufficient play, this being delivered in indian file by a subjacent feed line 2 of which at least the terminal part is in the form of a comb and acts together with a lifter 3 of matching shape which on command raises the product in line with the end wall of guide 1′, whereupon a thruster 4 which discharges the product from the lifter and inserts it into guide 1′operates at the appropriate time, using the working logic described above. 30 indicates known means which determine the dimensions of the product and transmit this information to machine processor 13, which also detects the rate of advance of conveyor 2, all so that the processor can automatically adjust the various working stages of the machine to the different dimensions of the products being packed.

Stretchable thermoplastic film 5 for wrapping the product reaches and is returned from the same extremity of mandrel 1 at which the product enters, being delivered from a spool 105 which is in a sufficiently convenient location for replacement when it is empty, for example above the packaging track, supported on a pair of parallel rollers 6, one of which idles while the other is driven by a drive unit 7 which unwinds the film from that spool. The film unwound from the spool is delivered to a buffer device which includes a set of rollers 108 which are mounted so as to idle on an appendage of supporting structure 9 which also supports said rollers 6 and comprises a set of rollers 8 mounted on a swinging arm 10 pivoted on said structure 9 at 11 and which has a transducer 12 on the pivot which detects the angular position of the swinging arm and converts it into an electrical signal which is used by process unit 13 to control motor 7 so that there is always a sufficient quantity of film in the buffer device to render the film wrapping stage independent of spool 105 from which the film is fed to tube-forming mandrel 1 through the action of a pair of rollers 15, 115 rotatably mounted on an appendage 109 of said structure 9 located above mandrel 1, at an appropriate distance therefrom and for example approximately on the vertical of the discharge opening from that mandrel. First roller 115 idles and is pressed by resilient means 16 towards the other roller 15 which is preferably rubber coated and positioned in relation to the upstream roller and mandrel 1 in such a way that it comes into contact with a suitable contact surface area of film, this roller being positively connected to a motor 17 under electronic speed and phase control, for example a stepping, brushless or other suitable type of motor controlled by processor 13. Suitable means, which are not illustrated, are provided so that roller 115 can be temporarily moved away from roller 15 so that the film can be inserted between them at the start of work.

Film 5 runs along mandrel 1 longitudinally in the same direction of advance as product P and through that mandrel is shaped in such a way as to form a tube which envelops tube guide 1′, while the edges of the film overlap, for example at the bottom of mandrel 1, where a device 18 of any type, for example a jet of hot air, which welds the said edges together longitudinally with at least one longitudinal weld which is preferably continuous and leak-tight, is in operation.

The anterior extremity of tube of film 5 leaving mandrel 1 is welded transversely and is held alternately by jaws 19, 119 which clamp it, form two welds and cut between them, comprising two matching and mutually overlapping parts connected to means diagrammatically indicated by arrows 20, 120 and 21, 121 controlled by processor 13, which on command moves them closer together and further apart with a self-centering movement which causes them to move horizontally away from and towards mandrel 1. The lower components of jaws 19, 119 have attached to them on the side opposite that facing mandrel 1 a corresponding projecting plane 22, 122 in the form of a comb. A motor-driven conveyor 23, formed by parallel and horizontal belts, at least in its initial part, through which the tines of planes 22, 122 of jaws 19, 119 can be inserted without interference, are provided at a suitable distance in line with and downstream from mandrel 1.

Bars 24, 124 which deliver hot air at the appropriate temperature to the wrapping film leaving the mandrel are located immediately downstream from mandrel 1. These components, which have already been described in the patent cited in the foreword, together with welding device 18 and other components which subject the film or packaging to heat treatment, are also controlled by processor 13.

The operation of the improved packaging machine differs, as previously stated, from that described in the preceding patents in the following way. While jaw 19, which is initially close to forming mandrel 1 and has closed upon the end of the tubular wrapping of the film produced by the said mandrel, begins to move away therefrom and while thruster 4 inserts product P which has to be packed within the said tubular wrapping of film leaving said mandrel 1, motor 17 is also active and delivers film at a linear rate which is suitably less than the rate of lateral movement of the clamp, and this difference in speed determines the percentage longitudinal stretching of the film which causes the tubular wrapping to elongate and shrink transversely to adhere intimately to product P through the particular elastic/plastic reaction properties of the film used. Processor 13 adjusts the cycle described to the dimensions of product P being packed, in particular its length, which is detected through aforesaid means 30 and 2. In the diagram in FIG. 2, A indicates the lateral movement of the anterior stretching jaw, B the rotational movement of film-feeding unit 15, 115, 17 and C indicates the sealing movement of the posterior jaw. These movements are inactive in the zero position and are instead active in the top position indicated by 1 in the ordinates. In the situation in FIG. 2, lateral movement A of the anterior jaw begins at the same time as activation of film-feeding unit 15, 115 and the difference in speed between the two movements determines the percentage stretching of the film, which is usually over 100%. During this stage motor 17 acts as a brake, or better applies changing friction, controlling the stretching of the film but without causing the latter to stop on forming mandrel 1, with the advantage that it assists the operation of welding unit 18, which can now operate substantially continuously and with the advantage that it prevents film stopping on the mandrel, thus preventing friction on first detachment and consequent local tensions in the film and unsightliness in the packaging, as was on the contrary the case in the prior art. In the example in FIG. 2, lateral movement A of the jaw ceases in phase with the rotational movement B of rollers 15, 115, but is reactivated in proper phase with sealing movement C of jaw 119 on the tail end of forming packaging Z. In fact at the end of the cycle, while the downstream welding jaw is closing, longitudinal stretching of the film must be small to permit the film to move on forming mandrel 1, in that the closing of this jaw will bring about elongation of the film. During this stage rollers 15, 115 and motor 17 continue to feed a little film to the forming mandrel in a controlled way to reduce longitudinal stretching during posterior closure and welding of the said packaging formed, by posterior jaw 119. The quantity of material which is actively delivered during this last stage depends in a directly proportional way on the height of product P, and this is brought about through processor 13 which has determined the dimensions of the product which has to be packed using aforesaid means 30 and 2.

The use of motor 17 with controlled speed and phase makes possible easy control of both the position and speed of film feed rollers 15, 115 to ensure that the advance of the film is in phase with respect to the jaw movements through the position adjustment control loop and the difference in linear feed between said feed rollers 15, 115 and the movements of the said jaws through the speed adjustment loop, all in such a way as to ensure good control of film stretching.

The example in the diagram in FIG. 3 differs from the preceding situation in FIG. 2 in that movement B of film feed rollers 15, 115 begins after lateral movement A of the upstream jaw and stops before the said movement A so that the film packaging is stretched by a greater amount and adheres to product P. It is obvious that at the start of the next packaging cycle, when the film has to be unwound for the next pack, the film will be under greater tension. In both the situations in FIGS. 2 and 3 the stopping of movement B activating rollers 15, 115 can be delayed, as illustrated for example in the diagram in FIG. 4, or its start may be brought forward with respect to movements A and/or C of the jaws, as illustrated in the diagram in FIG. 5. In particular if the second part of movement B of rollers 15, 115, that which takes place simultaneously with closing movement C of the downstream jaw, is particularly long, for example as in FIG. 5, this has the effect that the film is particularly relaxed at the start of the unwinding cycle.

Similar effects can be achieved as illustrated in the diagram in FIG. 6 by making use of control of the speed of feed rollers 15, 115, in the sense that this speed can be progressively increased towards the end of the unwinding cycle in such a way that the tube of wrapping film for the product is well tensioned at the start and then progressively becomes slower or more relaxed, with beneficial effects on the risk of film breakage at high packaging speeds, particularly during the initial stage of stretching by the upstream jaw. 

1) Machine for the packaging of products with stretchable thermoplastic film characterised in that it comprises: means (1) for making the wrapping film unwind from a spool into a tube and means (18) for longitudinally welding the overlapping edges of that film so as to produce a continuous tubular wrapping, means (15, 115, 17) to feed the film unwound from the spool to the said tube-forming means (1) in a controlled way, jaw means (19, 119) located downstream from the film-tube-forming means (1) which alternate in transverse gripping of the head and tail ends of the tubular wrapping leaving the said tube-forming means, and into which the product being packed is inserted at the appropriate stage, these jaw means being designed to support and cause the said wrapping to advance longitudinally and produce two transverse parallel leaktight welds therein, a transverse cut between the two welds and to hold the wrapping upstream and downstream from the two welded areas separated by the cut, means for cyclically inserting a product (P) through the said tube-forming means and for accompanying it as it exits therefrom and inserting it into the tubular wrapping being formed which is held anteriorly by one of the said jaw means (19, 119), means to ensure that the wrapping in the stage of being formed through the said tube-forming and longitudinal welding means (1) is subjected to longitudinal stretching in such a way as to adhere intimately to the product in a transverse direction, this condition being ensured by means which bring about a particular difference in speed between the jaw (19, 119) which supports the tubular wrapping and causes it to advance, and the said means (15, 115, 17) which feed the film to the tube-forming means (1), which also remain suitably active during the subsequent stage during which the tubular wrapping is gripped posteriorly by the appropriate posterior sealing jaw (119, 19), all in such a way that the wrapping film moves with an almost continuous movement over the said tube-forming means to prevent local unsightliness in the packaging and to assist operation of the longitudinal welding means (18) which are then able to operate substantially continuously, means (23) for collecting and removing the packs (Z) which are cyclically formed. 2) Machine according to claim 1), in which the film (5) is unwound from the spool (105) by means which are expressly dedicated to that function and which are capable of constantly providing the necessary suitably stretched quantity of film to the subsequent means (15, 115, 17) for controlled delivery of that film to the tube-forming means (1), motor-driven means (6, 7) being provided for rotation of the spool (105) and subsequent swinging arm means (8, 108, 10) for buffer storage of the cyclically unwound film, and means (12) which detect the angular position of the swinging arm and consequently the quantity of buffer film to produce a signal which is useful for control of the said means (6, 7) for unwinding film from the spool. 3) Machine according to claim 1), in which the said means for the controlled feed of film to the tube-forming means (1) comprise at least one roller (15) with a high coefficient of friction in relation to the film, for example a rubber-coated roller, on which the film is held and delivered with an extensive contact surface area by at least one parallel opposing roller (115), which idles and can be removed when necessary, the said rubber-coated roller (15) being connected to controlled rotation means which comprise a motor (17) under electronic control of speed and phase, such as for example a stepping motor, governed by a main processor (13) of the machine which operates it in proper phase with the lateral and closure movements of the jaws (19, 119), which are themselves also controlled by the said processor. 4) Machine according to claim 3), in which the processor (13) detects the dimensions of the product being packed through suitably dedicated means (30, 2) and suitably automatically adjusts the function of the said film feed means (15, 115, 17) and the operation of the jaws (19, 119) drawing, supporting, welding and transversely cutting the wrappings to those dimensions. 5) Process for the packaging of products with stretchable thermoplastic film, characterised by the following sequence of stages: formation of a tube of stretchable thermoplastic film unwound from a corresponding feed spool, with welding along its longitudinal edges in such a way as to form a continuous tube which is positioned almost horizontally and has a transverse cross-section of sufficient size for the product which is being packed to be inserted therein, closure of the head end of the said tube of film leaving the tube-forming and longitudinal welding stage with at least one transverse weld and support of the closed head in such a way that the tube which is also supported by the tube-forming means remains in a substantially horizontal position, insertion of the product (P), which is usually placed in a tray, within the length of tubular wrapping which is sealed and supported anteriorly in such a way as to be close to that anterior end of the tubular wrapping, longitudinal stretching of the tubular wrapping while this is being created through the tube-forming and longitudinal welding means, as it leaves these means and while the product being packed is being inserted into the said wrapping, all in such a way that through the effect of the stretching and as a result also of the properties of the film used the tubular wrapping shrinks transversely onto the product and wraps it intimately, closure of the posterior extremity of the longitudinally stretched tubular wrapping downstream from the product packed therein by at least one transverse weld, sealing of the end of the tubular wrapping following the packaging formed with at least one other transverse weld, transverse cutting of the tubular wrapping in the area between the said two transverse welds, while the head end of the next tubular wrapping is held while the tail end of the packaging formed is also held, while such packaging is supported horizontally and while at the appropriate time the head end of the same packaging is released in such a way that the film forming it can react longitudinally to adhere intimately to the packed product in that direction too, release of the posterior extremity of the packaging formed from the clamp and removal of the same from the packaging area. 6) Process according to claim 5, in which the film unwound from the spool (105) by the dedicated means (6, 7, 8, 108, 10) is fed positively and in a controlled way from the appropriate means (15, 115, 17) to the subsequent tube-forming and longitudinal welding means (1, 18) both during the stage of formation and longitudinal stretching of the tubular wrapping by the jaw means (119, 19) operating on the head end of the packaging being formed, and during the stage of sealing by the jaw means (19, 119) which takes place on the tail end of the said packaging during the completion stage, stretching of the film being ensured by a suitable difference in speed between the said feed means (15, 115, 17) and the successive jaw means (19, 119) which operate upstream and downstream of the said means (1, 18) respectively. 7) Process according to claim 6) in which activation of the said means (15, 115, 17) feeding film to the subsequent tube-forming and longitudinal welding means is automatically correlated with the size of the product being packed. 8) Process according to claim 5), in which the operation of longitudinal welding of the tubular wrapping of film as it passes over the tube-forming mandrel (1) is continuous because the wrapping film runs over the said mandrel continuously during both every cycle of product packaging and the transition from one cycle to the next, provided that continuity in the cyclical feed of products to be packed is ensured upstream of the said mandrel. 9) Process according to claim 5), in which the lateral movement (A) of the jaw (19) supporting the head end of the packaging being formed can begin at the same time as the movement (B) of the unit (15, 115, 17) feeding the film and the difference in speed between the two movements controls the percentage stretching of the film, and the said first movement (A) can cease in phase with the said second movement (B) which is reactivated at the appropriate stage with the closing movement (C) of the jaw (119) acting on the tail end of the packaging being formed. 10) Process according to claim 5) in which the movement (B) of the film feed means (15, 115, 17) begins after lateral movement (A) of the upstream jaw and ends before that movement (A) so that the film wrapping is stretched to a greater extent and adheres to the product (P) and that the film is under greater tension at the beginning of the next packaging cycle when the film is unwound for the next pack. 11) Process according to claim 9), in which the movement (B) of the film feed means (15, 115, 17) can be stopped with a delay in relation to the stopping of closing movement (C) of the jaw acting on the tail end of the packaging at the completion stage and/or can begin in advance of the horizontal and/or vertical movements (A and/or C) of the jaws (19, 119). 12) Process according to claims 10), in which the movement (B) of the film feed means (15, 115, 17) can be stopped with a delay in relation to the stopping of closing movement (C) of the jaw acting on the tail end of the packaging at the completion stage and/or can begin in advance of the horizontal and/or vertical movements (A and/or C) of the jaws (19, 119). 13) Process according to claim 5), in which the speed of movement (B) of the film feeding means (15, 115, 17) can be progressively increased towards the end of the wrapping cycle in such a way that the tube of film wrapping the product is firmly tensioned at the start and then progressively becomes slower or more relaxed, with beneficial effects on the risk of film breakage at high packaging speeds, particularly during the stage of the start of stretching by the jaw acting on the head end of the wrapping. 